Boris Johnson may condemn violence and antisocial behaviour in his day job as mayor of London, but he is not averse to expressing a rather violent desire to "end what is left of my political career with one almighty head-butt" levelled at Ed Balls, the schools secretary.

Johnson's tirade against Balls was a response to what the Conservative mayor described as "death-defyingly stupid" comments from Balls on the subject of Latin in schools.

"There are times when a minister says something so maddening, so death-defyingly stupid, that I am glad not to be in the same room in case I should reach out, grab his tie, and end what is left of my political career with one almighty head-butt," said the mayor, renowned as a passionate classicist. "Such were my feelings on reading Mr Ed Balls on the subject of teaching Latin in schools."

Johnson's comments, provoked by the minister's claim that "very few parents" are pushing for Latin in state schools, were not blurted out in a red mist moment, but crafted for his Daily Telegraph column, for which he earns £250,000 a year.

Is this fighting talk fitting for such a key Tory figure?

Conservative Central Office did as it often does when the mayor says or does something potentially embarrassing and declined to comment, saying it was a matter for City Hall, not them – which is strange since the mayor is spending his spare time at the moment visiting London constituencies with Tory candidates.

Emily Thornberry, a north London MP, appeared to be more shocked by the mayor's priorities than his colourful language.

"Perhaps if Boris Johnson cared as much the pressing needs of London as he does about Latin there would be lower bus fares, no cuts to police numbers, no cuts to tube ticket office staff and a much smaller list of broken manifesto commitments," she said.